Filter



Nov. 2, 1937. w HANSELL 2,097,458

FILTER Original Filed Sept. 24, 1951 CIICOOICOOI l A INVENTORv l 6656 I CLARENQE W,HANSELL f1 f Mk ATTORNEY Patented Nov. 2, 1937 UNITED STATES FILTER Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware 10 Claims.

This invention relates to a filtering system and is especially directed to filters utilizing resonant means or elements such as piezo-electric crystals.

This application is a division of my copending application Serial No. 564,770, led September 24, 1931 and patented May 14, 19351as U. S. Patent No. 2,001,387.

It has been proposed to use a piezo-electric crystal as a filter, but it has been found under certain circumstances that such a iilter, especially when neutralized in accordance with my invention described in my copending application Serial Number 203,901, led July 7, 1927 and patented June 18, 1935 as U. S. Patent No. 2,005,083, does not pass a sufficiently Wide brand of frequencies. I found that the simple paralleling of crystals having overlapping resonance curves for obtaining a band pass filter, does not present all that is to be desired, forat frequencies midway between the resonance points of the two crystals it will be found that one crystal acts as an eifective inductance and the other an effective capacity. Consequently, the pair of crystals act as an anti-resonant circuit preventing a transfer of energy at a desired energy passing frequency.

To overcome these defects is the principal object of my present invention and to do so I reverse the polarity of voltages applied to, in general, resonant elements or systems such as crystals with overlapping resonance curves, so that the currents through the elements or crystals wl no longer oppose but will add for all frequencies between their resonance frequencies. At the same time, for frequencies lying on the same side of their resonance curves but well up on the resonance curve of one element or crystal there is little cancellatio-n of the passed currents for the reason that the current through one of the crystals or elements is much larger than the others. Beyond the desired pass band the crystal currents of course cancel more completely and the suppression of undesired frequencies is accomplished.

A further object of my present invention is to provide, in combination with a side band telephony system, an improved crystal or other resonant element filter as just briefly described. In this manner, for single side band telephony, a lesser number of modulation stages is needed, for the reason that the filter will operate satisfactorily with much smaller percentage differences in the frequency of the side bands than heretofore found possible.

Another object of my present invention is to provide, in combination with a radio receiver,

an improved crystal filter which will permit improved frequency selectivity `and more efficient reception than heretofore possible.

Still another object of my present invention is to provide for the neutralization of the interelectrode capacitiesof the crystals used in my improved band pass filter. In this manneramuch sharper band pass and more complete suppression of undesired frequencies is obtained for the filtering system. Briefly, in order to accomplish this I use the interelectrode capacity of one crystal, or l.that interelectrode capacity supplemented by an yadditional capacity to neutralize the interelectrode capacity of the other crystal and vice versa.

In order to sharpen the characteristic of ordinary lters it is a further object of my vpresent invention to provide means, preferably a filter 'circuit according, to my present invention in parallel with the ordinary filter circuit whereby its characteristic may be improved.

Although I have dened my invention Vas required by law With particularity .in the .appended claims, it may best be understood, both as to its structural organization and mode of operation', by referring to the accompanying drawing wherein t Figure 1 is a schematicr wiring diagram `of a single side band transmitter incorporating therein 4my improved band pass crystal filter;

Figure 2 is a diagram of a ,pair of series `connected condensers .and piezo-electric crystals employed as a lter; Y l

Figure 3 is a diagram of parallel arranged piezo-electric crystals employed filters;

Figure 4 is a diagram of Va band pass -filter applied to a receiving circuit having a plurality of interposed piezo-electric crystals employing four electrodes.

Turning to Figure 1, a relatively low-.frequency carrier suppression modulatorZ is supplied with modulatingy potentials from a suitable source 4 and with carrier energy from a carrier source 6. Single side band energy from the carrier suppression .modulator 2 is passed by a band pass filter and undesired frequencies are rejected. The-operation is well .known in `the art Aand need not be repeated here. The single side band output of this modulator is fed to a second carrier suppression modulator 8 supplied with high-frequency carrier energy from va source Hl.

By the use of my improved Vband pass filter I2, whose structure and operation will be described more fullyhereinafter, a. single side band is 'sep-- arated from the side bands appearing in'theout- Lio cycles requires at least three stages of carrier suppression modulation in order that the percentage difference between thedesired side band and the suppressed carrier and other 'side band energies might be suiicient for obtainable lters to discriminate between them. Since my new lter circuit is much more selective than previously known band pass lters it permits one` stage of modulator to take the place of two or more of the stages previously required.V

My improved band pass filter comprises a plurality of resonant or oscillating elements, here in the form Vof, piezo-electric crystals I8, 20 like electrodes 22,- 24 of which are connected serially in phase opposition Vor in reversed polarity by means of an impedance 2B here in the form of a broadly tuned circuit comprising reactor or inductor 28 and variable reactor or capacitor 30. The other electrodes 30, 32 of the band pass lter are conductively connected by connection 34. Between connection 34 and a point of substantiallyV zero alternating potential or a point substantially at the midpoint of alternating current circuit 26 there is connected an alternatingcurrent output circuit 36 comprising inductance 38 and variable capacity 4B.

By virtue of the connections just described,

input energy isv applied to theV crystals in reversed phase as a result of which, for the mean frequency to be passed by crystals I8, 28, ground to have overlapping resonance curves, their outputs add; whereas it is to be noted if theywere simply connected in parallel, themean frequency desired to be passed wouldnot be passed.Y For some frequencies within the desired pass band whichV lie on the same side of the resonanceV curves of both crystals, there is some cancellation, but this not serious for the reason that by virtue of the resonance characteristics of the crystals in this region one crystal will be passing a large amount of current; whereas the other will be passing aV relatively small amount ofY current. Hence, the pass band is almost as wide as the sum ofthe pass bands of the two 'v individual crystals.

By virtue of the connection ofthe output circuit it should be clear that by suitable spacing of the electrodes of the crystals I8, 20, the transfer of energy outside the resonance bands of the crystals themselves may be prevented by making the interelectrode capacity of one crystal neutralize the interelectrode capacity of the other cuit to the point near the center of coil 28.

Of course, as already indicated, adjustment of the crystal electrodesV alone may be suicient to establish complete neutralization, and Aas means for varying thespacing and area ofl crystal elec-V These condensers, of course,

trodes are well known in the art, they need not be described in detail here.

Y The filtering circuit shown, as will occur readily to one skilled in the art, may be varied in many ways without departing from the spirit of the present invention. Thus, for example, as shown in Figure 2, the alternating current circuit 26 may be replaced by a pair of series connected condensers 46, 48, preferably of about the same size and between which and the conductive con- `nection 34 there is connected an alternating circuit 58 here in the form of a single inductance coil. rI'he impedance formed by condensers 46, 48 may also be connected through blocking condensers 52, 54 to some other alternating current circuit 56, and neutralization may be accomplished by varying the spacing of electrodes 24, 22 of crystalsv I8, 20, or by varying the relative capacities of condensers 46 and 48.

Although, in Figure 1, I have shown only two crystals by way of illustration, it should be noted that my invention includes the use of any number of crystals having successively overlapping resonance curves in my filter circuit. If more than two crystals are used, thenthose with succ'essively overlapping resonance curves are connected with reversed polarity in the manner shown for the two crystals.

In designing the filter I may obtain a very narrow pass band and large ratio between the energies passed f or desired and undesired frequencies by. using low impedance input and output circuits for the filter. In other cases, where a broader pass band for each crystal is desired with a minimum number of crystals, I may use high impedance input and output circuits, thoughV in this case the ratio of energies passedY at desired and undesired frequencies is decreased.

It should be understood that the input and output ends of the lter may be reversed without changing its characteristics. In other Words, it is a band pass filter for energy passing through itin either direction. Y

As Vpreviously, stated, if a wider pass band is desired, any Vnumber of crystals with successively overlapping resonance curves may be used, provided crystals ground for adjacent frequencies are connected to opposite sides of the lter input circuit. .-Thus, as shown in Figure 3, if it is desired to increase the frequency band width on the side of the mean frequency passed and on the side of the mean frequency towards which the resonance curve of crystalY I8 lies for the greater part, then an additional crystal 'I9 may be added, connected as shown to the opposite side of the input circuit 2l, opposite inthe sense that it is opposite the side of the inductance coil 2| to which crystal I8 is connected.

V Similarly, if it is desired to increase the frequency band width in the direction of the frequencies passed by crystal 20, an additional crystal 23 having a characteristic overlapping that of crystal 28 may be added connected to the side opposite input circuit 2 I to that to which crystal` 20 is connected. Neutralization may be added if found desirable by adjustment of the Various crystal electrodes', by the addition of supplementary trimming condensers 25, 21, or by varying the relative voltages applied to the two branches of thelter.

If desired, my invention may be carried out with crystals having more than two electrodes. Figure (l shows Oneway of using crystals with lfour electrodes. In this case no neutralization is required, particularlyiif the circuits are shielded.

More particularly, in Figure 4 the crystals 6B having overlapping resonance `curves in accordance with what has gone hereinbefore are each provided with pairs of electrodes 62, i betweenV which the crystals constitute a resonant means of electrical coupling through the piezo-electric effect and the mechanical Vibration of the crystals. Capacitive feed of energy between the two circuits 6B, 68 coupled by the filter including crystal element 69 is prevented by the use of suitable electrostatic shielding indicated diagrammatically at lll. From what has been said hereinbefore the operation of the system shown in Figur-e 4 should be obvious and therefore requires no further discussion.

In Figures l and 4 the tuning of the input and output circuits for the filters shown not only assist in obtaining the best impedances for use with a filter, but it also suppresses undesired frequencies. The electro-mechanical oscillators such as the piezo-electric crystals, and also resonant transmission lines, respond to a series of frequencies and exhibit resonance at frequencies approximately at harmonic relation, the tuned input and output circuits do not respond to harmonies of their fundamental frequencies, and so suppress undesired frequencies which would otherwise pass through the filter. Thus, I broadly combine tuned circuits having only one resonant frequency with circuits or mechanically oscillated equivalents of circuits which tune sharply for a series of frequencies to obtain a filter with shar) resonant discrimination but only one pass As indicated hereinbefore, a still further use for my invention is to sharpen up the cut-olf characteristics of the more common types of filters. This may be done by connecting one of my electro-mechanical filters in parallel with an ordinary filter. Then, if the pass band of the electro-mechanical filter lies in the imperfect cut-off region of the ordinary filter, it will extend the pass band of the ordinary lter and the cutoff characteristics of the combination will be that of the electro-mechanical filter, which is very sharp, while the width of pass band will be substantially that of the ordinary filter.

In the crystal lter of Figure 4, I may obtain the desired results by varying the sizes of the crystal electrodes -o-r by varying the coupling to either input or output electrodes, or both. In fact, any method for varying the effectiveness of the individual vibrators, including the application of damping, may be used whereby the combination of filters gives the desired `characteristic.

In my claims where I speak of connecting the filter circuit elements cophasally or in phase opposition, itis to be understood that I refer to the phase conditions which would correspond to resonance of the elements. Since the elements will not be resonant for the same frequencies and their currents change phase as the frequency is changed, it must be understood that I have used the expressions cophasally and in phase opposition for want of more expressive language.

Having thus described my invention, what I claim is:

l. A crystal filter comprising a plurality of crystals having overlapping resonance curves, means connecting an electrode of each of the crystals directly together, and means connecting other electrodes of thel crystals in phase .oppo-V the midpointof said'reactor and said conductive connection. Y Y

3. A piezo-electric crystal'filter comprising a pair of piezo-electric crystals having overlapping resonance curves, an impedance connected between electrodes of said crystals, a conductive connection connecting together other electro-desV of said crystals, an input circuit coupled to said impedance, an output circuit connectedy between the midpoint of said impedance and said conductive connection, the interelectrode capacity ofk said crystals being variable whereby the interelectrode capacity of one crystal may be adjusted to neutralize the interelectrode capacity, of the other crystal.

4. A piezo-electric crystal'filter comprising an input source of current supply having a plurality of different desired resonant frequencies, a pair of piezo-electric crystals, said crystals having symmetrically displaced overlapping resonant currents of similar characteristics as said desired resonant frequencies, an inductance coil connected serially between a pair of electrodes of said crystals, a conductive connection connecting other electrodes of said crystals together conductively, an alternating current circuit coupled to said inductance coil, an alternating current circuit connected to the midpoint of said coil and to said conductive connection, means for varying the capacity between electrodes of one of said zoY crystals, and means for varying the capacity ben said impedance connection, said midpoint connection Aeffectively directly connecting together the electrodes of said crystals to an output inductance of said filter.

6. A transmitting system comprising a carrier suppression modulating system, a band pass piezoelectric filter coupled to the output circuit of said carrier suppression system, and a power amplifier coupled to the output circuit of said band pass filter, said filter comprising'a pair of piezo-electric crystals having a pair of electrodes connected in phase opposition for applied alternating currents, said crystals having overlapping resonance curves, an output circuit connected to the other electrodes of said crystals and'to subfstantially the midpoint of the connection connecting said electrodes in phase opposition, and

means for neutralizing the interelectrode capac-v n plurality of piezo-electric crystals having overlapping resonance curves, each of said crystals having a plurality of pairs of electrodes, means Y for connecting like pairs of electrodes in parallel, means for connecting the other pairs of electrodes of adjacent resonant frequencies, in reversed polarity, circuits connected to said connection means, and shielding means for preventing coupling of said circuits other than that through the crystals. Y Y

8. A crystal lter comprising a pair of piezoelectric crystals having overlapping resonance curves, a reactor connecting an relectrode of each of the crystals in phase opposition With respect to an electrode of one of the other crystals, means conductively connecting the other electrodes of the crystals together, an input circuit coupled to saidV reactor, and an output circuit connected between the midpoint of said reacto and said conductive connection.

9. A crystal lter comprising a pair of piezoelectric crystals having overlapping resonance curves, a first reactor connecting an electrode of each of the crystals in phase opposition with respect to an electrode of one of the other crys-V tals, a second reactor connecting the other electrodes of the crystals together, and to a point on said first reactor, an input circuit coupled to one of said reactors, and an output circuit coupled to the other of said reactors.

10. A crystal lter comprising a plurality of crystals having overlapping resonance curves, a rst reactor connecting an electrode of each of the crystals'in phase opposition with respect to an electrode of one of the other crystals, a second reactor connecting the other electrodes of the crystals together and to a point on said first reactor, an input circuit coupled to one of said reactors, and an output circuit coupled to the other of said reactors.

CLARENCE W. HANSELL. 

